Liquid crystal display devices have various advantages such as low power consumption, light weight, and slim profile. Owing to these advantages, they have taken the place of conventional CRT displays in various fields. Particularly, liquid crystal display devices are essential components in mobile devices having spread rapidly in recent years, such as mobile phones or smartphones.
In a known structure of such liquid crystal displays, a pair of polarizers are arranged on a backlight source in a crossed Nicol state on a viewer side and a backlight source side of a liquid crystal cell.
In a liquid crystal display device having such a structure, light emitted from the backlight source passes through the polarizer on the backlight source side, the liquid crystal cell, and the polarizer on the viewer side, whereby an image is displayed on a display screen.
Commonly, the polarizer has a layered structure including a polarizing element and a light-transmitting substrate, and the light-transmitting substrate of the polarizer is a film of cellulose esters typified by triacetylcellulose (see Patent Literature 1). Cellulose esters are often used because of the following advantages. Since cellulose esters are excellent in transparency and optical isotropy to hardly have an in-plane phase difference (retardation value is low), they scarcely changes the vibration direction of the incident linear polarized light to be less likely to affect the display quality of the liquid crystal display device. In addition, since they have appropriate water permeability, moisture contents remaining in a polarizing element upon production of the polarizer can be dried through an optical layered body.
Cellulose ester films, however, have too high moisture permeability to problematically cause reduction in the degree of polarization due to discoloration in a moisture resistance test. Because of such a problem of cellulose ester films, a protective film to be used is desired to be a versatile film that is excellent in transparency, heat resistance, and mechanical strength, and is readily available at lower costs than that of cellulose ester films or producible by a simple method. For example, an attempt is made to use polyester films such as polyethylene terephthalate films instead of cellulose ester films (see Patent Literature 2).
In a liquid crystal display device having such a structure, efficient transmission of light from a backlight source to a display screen is important to improve the luminance of the display screen. Particularly, in mobile devices such as smartphones having spread rapidly in recent years, more efficient transmission of light from a backlight source to a display screen is desired as it directly influences the battery duration time.
In a known example of such a liquid crystal display device, a polarization separation film is provided between a backlight source and a polarizer on the backlight source side to allow polarized light to be incident on the polarizer on the backlight source side, thereby improving the luminance of the display screen. The polarization separation film allows specific polarization components to pass therethrough and reflects the other polarization components back to the backlight source side.
However, if a polarizer on the backlight source side in a liquid crystal display device having such a structure includes a protective film formed of a polyester film, the transmittance may lower. Since a polyester film has a high-polarizability aromatic ring in its molecular chain and thus has very high intrinsic birefringence. Accordingly, the polyester film has a characteristic of easily exhibiting birefringence accompanying orientation of the molecular chain as a result of stretching treatment for imparting excellent transparency, heat resistance, and mechanical strength.
Accordingly, if a polarizer that includes a light-transmitting substrate having in-plane birefringence, such as a polyester film, is used as a polarizer on the backlight unit side in a liquid crystal display device, the polarization state of specific polarization components having passed through the polarization separation film may change to lower the transmittance.